The present invention relates to a frequency-modulated continuous-wave (FM-CW) radar system and, more particularly, to an FM-CW radar system that performs processing for detecting the distance and relative velocity of a target object by variably setting the target object detection range.
An FM-CW radar system is mounted, for example, on a vehicle or the like, and measures the distance or relative velocity of a target object, such as a vehicle traveling in front, by transmitting a continuous wave frequency-modulated with a triangular wave. The distance or the relative velocity is measured in the following manner.
First, the transmitted wave from the radar is reflected by a vehicle traveling in front, and a beat signal (radar signal) occurring between the received signal of the reflected wave and the transmitted signal is obtained. This beat signal is fast-Fourier transformed (FFT) to analyze the frequency. The frequency-analyzed beat signal exhibits a peak at which the intensity becomes large corresponding to the target object. The peak frequency corresponding to this peak carries information concerning the distance, and the peak frequency differs between the up portion and down portion of the triangular FM-CW wave due to the Doppler effect associated with the relative velocity of the vehicle in front.
The distance and relative velocity of the vehicle in front are determined from the peak frequencies in the up and down portions of the triangular wave. If there is more than one vehicle traveling in front, one pair of peak frequencies, one in the up portion and the other in the down portion, occurs for each vehicle. The pairing of the peak frequencies between the up and down portions is done on a vehicle-by-vehicle basis.
As described above, in the prior art FM-CW radar system, the pairing is performed based on the peak frequencies obtained by applying signal processing to the transmission/reception beat signal, and the distance and relative velocity of the corresponding target object are determined; in reality, however, many target objects are detected and, if signal processing is to be performed for all the detected objects, not only a long processing time but also a large memory capacity will be required.
Furthermore, the number of target detection outputs that the radar system can produce is limited by the data update rate and communication baud rate of the system. As a result, data necessary for another system, for example, a speed control system, must be output by selecting such data within the radar system, but the selection requires complicated processing.
Because of these limitations, coupled with the limited memory capacity, there arises the possibility that, depending on conditions, data relating to the intended target object may be discarded, causing an ill effect on, for example, the operation of the speed control system.
To address this problem, it is practiced in the prior art to limit the detection range of the FM-CW radar system and set the range to the distance necessary for the operation of the speed control system. Limiting the detection range, however, greatly affects the detection accuracy of the radar system in detecting the distance and relative velocity of the target object.
In view of this, it is an object of the present invention to provide an FM-CW radar system that can variably set the target object detection range, thereby reducing the memory capacity required within the system and improving the accuracy of target object detection.
To solve the above-described problem, according to the present invention, there is provided an FM-CW radar system which transmits an FM-CW wave and receives a reflected wave from a target object, comprising: modulating signal generating means capable of changing a modulating signal to be applied to the FM-CW wave; calculating means for calculating a distance or relative velocity with respect to the target object by performing processing for detection of the target object by fast-Fourier transforming a beat signal occurring between a transmitted signal of the FM-CW wave and a received signal of the reflected wave; and control means for determining a detection range based on the distance calculated by the calculating means, and for performing control for the modulating signal to be changed by the modulating signal generating means. Here, the modulating signal generating means is configured to be able to change the modulating signal by changing one parameter selected from among a modulation frequency, a triangular wave frequency, and a transmit wave center frequency associated with the FM-CW wave.
Provisions are also made so that, when two or more target objects are detected, the control means selects the shortest distance from among the distances of the detected target objects calculated by the calculating means and performs control to change the modulating signal so that the detection range becomes equal to a distance obtained by adding a prescribed distance to the selected distance, while when a vehicle equipped with the radar system is traveling around a curve, and when a detected target object can be judged to be a roadside fixed object, the control means performs control to change the modulating signal so that the detection range becomes equal to a distance obtained by subtracting a prescribed distance from the distance of the target object calculated by the calculating means.
Further, the FM-CW wave from the FM-CW radar system is transmitted as a plurality of beams over a prescribed scanning angle, and the modulating signal for changing the detection range is changed for each of the beams.